Compact flat panel color calibration system

ABSTRACT

A compact flat panel color calibration system includes a lens prism optic able to pass a narrow, perpendicular, and uniform cone angle of incoming light to a spectrally non-selective photodetector. The calibration system also includes a microprocessor operable to determine the luminance of the display based upon the information gathered by the photodetector. A software module included in the calibration system is then operable to process the luminance information in order to adjust the flat panel display.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/053,068 filed Feb. 7, 2005 now U.S. Pat. No. 7,068,263 issued Jun.27, 2006, which is a continuation of U.S. application Ser. No.10/013,576 filed Dec. 10, 2001 now U.S. Pat. No. 6,853,387 issued Feb.7, 2006, which claims the benefit of U.S. Provisional Application No.60/254,432 filed Dec. 8, 2000.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of photodetectordevices and more particularly to a flat panel color calibration system.

BACKGROUND OF THE INVENTION

Conventional radiometric or calorimetric sensor systems employ eitherspectrally selective or nonselective photodetectors to quantify theluminance level of the light that reaches them. Such sensor systems havebeen used to calibrate Cathode Ray Tube (CRT) displays. To ensureaccuracy, these sensor systems must maintain the sensor elements incontact with or at a uniform fixed distance from the display. A commonimplementation of such a sensor system utilizes a suction cup in orderto firmly adhere the sensor to the CRT screen that has the additionaleffect of blocking out any ambient light from the sensor. For LiquidCrystal Displays (LCDs), however, the colorimetry of the display isdependent in part on the optical path length of the display which is theproduct of the birefringence of the liquid crystal material and the cellgap spacing. Since the glass of a LCD panel is relatively thin whencompared to that of a CRT, the force of the suction used to attach thattype of sensor would cause spacing changes that would change theluminous intensity of the light in that area and corrupt the accuracy ofthe very measurement that such a sensor would be attempting to make.

Unlike CRTs, both the luminance and the wavelength of light emitted fromLCD flat panels vary depending on the angle at which they are measuredor collected. This poses a problem for traditional color calibrationsensors that all assume any portion of the light measured isrepresentative of the whole. Therefore, it is desirable to provide asensor system capable of accurately calibrating LCD devices that doesnot alter the cell gap spacing and collects luminance informationorthogonal to the surface of the glass and with a narrow acceptanceangle.

SUMMARY OF THE INVENTION

From the foregoing it may be appreciated by those skilled in the artthat a need has arisen for a compact flat panel color calibrationsystem. In accordance with one preferred embodiment of the presentinvention, a system for calibrating the color of a flat panel display isprovided that substantially eliminates or greatly reduces disadvantagesand problems associated with conventional display calibrationtechniques.

According to an embodiment of the present invention, there is provided asystem for calibrating a display that includes a lens prism optic thatoperates to pass a narrow, perpendicular, and uniform cone angle ofincoming light, a photodetector, a microprocessor operable to determinethe luminance of the display in order to derive the color temperatureand firmware operable to adjust the display in accordance with themicroprocessor determination.

The present invention provides various technical advantages overconventional display calibration techniques. For example one technicaladvantage enables accurate measurements of a Liquid Crystal Display(LCD) by employing a lens prism optic that provides a narrow and uniformbeam of light to the photodetector system. Another technical advantageis to augment the calibration measurement by allowing the system tocompensate for the effects of ambient light that might be glaring offthe surface of the display. Other technical advantages may be readilyascertainable by those skilled in the art from the following figures,description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals represent like parts, in which:

FIG. 1 illustrates a simplified diagram of the environment of a compactflat panel color calibration system;

FIG. 2 illustrates an assembly view of a color calibration sensoraccording to one embodiment of the present invention;

FIG. 3 illustrates a circuit schematic for an electronic filter in thecolor calibration sensor system; and

FIG. 4 illustrates a simplified diagram of software modules thatinteract with the color calibration sensor;

FIGS. 5A-H illustrate screen displays provided by the software modules.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a flat panel color calibration system 100. Flat panel colorcalibration system 100 includes a sensor system 102, a backlit LiquidCrystal Module 104, a host computer 106, and a flat panel monitor 108associated with liquid crystal module 104. Typically, flat panel monitor108 and liquid crystal module 104 are part of a single display device.Sensor system 102 measures luminance information emitted by liquidcrystal module 104 and provides the luminance information to thecalibration software module 172 residing within host computer 106 overan I2C serial interface portion of a video signal cable, RS 232, orUniversal Serial Bus (USB) connection 109. Host computer 106 includes adigital interface port 112 for receiving the luminance information fromsensor system 102, and either a serial port 114 or a graphics interfaceport 116 for relaying commands to flat panel monitor 108. Thecalibration software application within host computer 106 then providescalculated color adjustments based on the luminance information receivedfrom sensor system 102 to flat panel monitor 108. Flat panel monitor 108includes a scaler board 118 for receiving color correction informationfrom the calibration software application within host computer 106. Flatpanel monitor 108 also includes an inverter unit 120 for driving a pairof top (red) lamps 150 and a pair of bottom (blue) lamps 152 of liquidcrystal module 104. A LCD control board 122 provides the signals tocontrol gate drivers 154 and source drivers 156 of liquid crystal module104. An on-screen display (OSD) board 126 provides a graphical userinterface driven by mechanical push button controls for manualadjustment of flat panel monitor 108. OSD board 126 also includes avisual control board that provides appropriate visual control signalsfor flat panel monitor 108.

Scaler board 118 of Flat Panel Monitor 108 includes an Inter-IntegratedControl (I2C) input interface 132 from host computer 106 to manageseveral components via independent buses. These include a ColorLockmicrocontroller 140 which handles executive control of color managementfunctions, a backlight inverter 120 accessed through an I²C interface130, and an Extended Display Identification (EDID) EEPROM 142 whichcontains calorimetric profile information for the specific liquidcrystal module 104. A Transition Minimized Differential Signal (TMDS)Receiver 134 accepts video information from graphics interface port 116residing within host computer 106 and forwards it to a gmZ3 scaler chip144 which adapts the input graphics into a resolution format compatiblewith liquid crystal module 104. A Field Programmable Gate Array (FPGA)device 146 multiplexes user inputs from the OSD board 126 through EEPROMdevice 142 onto the main display data path via CMOS device 148 toprovide menu-driven settings of various liquid crystal module 104functions. FPGA device 146 also controls manual dimming of the backlightfrom an OSD panel 142 by controlling inverter 120 through I2C interface130. In one embodiment, control data and control commands are receivedinto a level-shifter serial port 136 from a serial output 114 withinhost computer 106. In this embodiment, serial port 136 is RS-232 but mayalso be of the Universal Serial Bus (USB) type. 12V system power isprovided through an input connector 138 for an external plug 124.

Flat panel monitor 108 supports non-native resolutions through anoperation known as scaling. Scaling adapts the input graphics mode intoa resolution that meets the stringent timing requirements of the VESAtiming specification. Flat panel monitor 108 has the ability to adjustthe displayed image automatically for the user. In general, the embeddedscaler chip 144 analyzes the input timing provided by host graphics card116 through TMDS receiver 134 and adjusts the liquid crystal module 104to optimal image quality. These extensive scaling and image coloradjustment capabilities demand an intuitive user interface. On a displayproduct, on-screen controls appear superimposed over the regular image.Executive color control in our embodiment resides within an 8051-basedmicrocontroller 140. Microcontroller 140 has an 8-bit 8051 architecturethat provides flexible general purpose I/O, built-in program memory,RAM, and other peripheral devices. The low cost of microcontroller 140is also appropriate for a single calibration technique Colorlock. FPGAdevice 146 multiplexes the OSD board 126 data onto the main displaydatapath. FPGA device 146 also controls backlight dimming for thesystem. This control is accomplished via SPI bus communication with themicrocontroller 140. Via an included EDID EEPROM 142, microcontroller140 is able to perform an I²C master read of this device and obtain thepanel's serial number for display on the information screen provided byOSD board 126. Certain parameters are stored on a global memory pagewhereas other settings are saved in timing mode-specific memory pages.

FIG. 2 shows the components of sensor system 102. A hanger 200 isoperable to position a photodetector 210 near a surface of backlitliquid crystal module 104. In a preferred embodiment, hanger 200 iscomposed of ABS plastic or any other material that is similarlylightweight and durable. Attached to hanger 200 is a housing 202.Housing 202 is compact, yet of sufficient size to contain photodetector210. Sensor system 102 also includes an elastomeric or foam cushion 212,various electrical circuit components fixedly attached to a circuitboard 214, a color filter 216, and a lens 208 operable to permit thepassage of light.

Photodetector 210 is comprised of a linear silicon photodetector, anexemplary type of which is an OPT101P photodiode manufactured byBurr-Brown. The OPT101P operates from +2.7V to +36V, has a highresponsivity of 0.45 A/W at 650 nm, and a quiescent current of 120 μA.Photodetector 210 is preferably spectrally non-selective and thus not acalorimeter. However, photodetector 210 can effectively function as acalorimeter because the flat panel make up of liquid crystal module 104has Red, Green, and Blue filtration on its light output. Photodetector210 works in conjunction with microcontroller 140 residing in flat panelmonitor 108 in accordance with software instructions from an applicationresiding in host computer 106 to calibrate liquid crystal module 104 bysetting its luminance, color temperature, and gamma profile.Photodetector 210, with biasing circuits and an on-chip transimpedanceamplifier whose output voltage increases linearly with light intensity,eliminates stray capacitance that can cause errors from leakage current,noise pick-up, and gain peaking. Photodetector 210 preferably operatesin a photoconductive mode to take advantage of excellent linearity andlow dark current. Photodetector 210 has a correlation to the actinicresponse of the human eye. Sensor system 102 is designed to perform itscalibration function at a signal level centered about a count of 235with a variance of +/−3 counts or about 1 sigma. This provides a 2 timesguardband, or a +/−7 count allowable variance, which equates to an errorof one ΔE*, the smallest change in three dimensional color spacedetectable by the human eye.

In a preferred embodiment, the characteristics of the linear siliconphotodetector include low sensitivity in the 400 nm (Blue) portion ofthe electromagnetic spectrum but a relatively (about six times) higherresponse in the 700 nm to 900 nm (Red to Infrared) portion. To level outthe sensitivity of the sensor element 210, color filter 216 is placedbetween sensor element 210 and lens 208. A narrow band green filter maybe used to emulate the green channel of the green primary of liquidcrystal module 104 in order to achieve speed and accuracy necessary toperform calorimetric calibrations for all R, G, and B channels. In apreferred embodiment of the invention, color filter 216 is comprised ofa Light Steel Blue #720 cyan filter manufactured by GAM. Color filter216 is operable to equalize the signal to level off the performance ofphotodetector 210 so that when photodetector 210 is calibrated tooperate in the Green spectral region, correct sensitivity levels aremaintained in the Red and Blue regions as well. The GAM Light Steel Blue#720 cyan filter 216 very nearly compensates for the OPT101'stransmission characteristics, both with a midpoint relative responsevalue of 0.35 at 580 nm. This results in a very uniform output responsefrom 440 nm to 640 nm. Should signal strength need to be furtherattenuated, multiple layers of the color filter 216 may be utilized. Forexample, two blue-green filters may be used, one for chromaticsensitivity and the other for setting the proper luminance signal range.

The linear silicon photodetector 210 will respond accordingly towhatever wavelength of light reaches it after passing through colorfilter 216. To obtain the most accurate measurements for colorcalibration purposes, the angle at which light is measured from a flatpanel liquid crystal module 104 must be kept small. Sensor system 102operates to provide a narrow and uniform cone angle of incoming lightwhich is presented to photodetector 210. This narrow and uniform coneangle enables sensor system 102 to accurately measure liquid crystalmodule 104 without the need to maintain a constant or critical distancefrom the surface of liquid crystal module 104. It also allows the sensorto be used in an off contact mode in order to measure the amount ofveiling glare on the front surface of liquid crystal module 104 so thatthe software application can compensate for its effect on an image. In apreferred embodiment, lens 208 is a solid optical lens system, comprisedof a front spherical surface through which the light passes in themanner of a Total Internal Reflection (TIR) 45-degree mirror. Lightpassing through lens 208 is turned to pass through the side of lens 208and through the color filter 216 to photodetector 210. In a preferredembodiment, photodetector 210 is located at the infinity focus of lens208. In one implementation, the lens 208 is injection molded frompolymethylmethacrylate (PMMA) plastic material and has an index ofrefraction of 1.489.

By implementing lens 208 in this manner, a narrow and radiometricallyconstant measurement geometry of incoming light is presented tophotodetector 210. Lens 208 provides a long focal length with arelatively small head configuration to keep the effective viewing angleat plus or minus two degrees (±2°) regardless of the distance of sensorsystem 102 or its attitude with respect to the front surface of liquidcrystal module 104. Advantages made possible by this design include asensor system 102 being able to make more accurate and enhancedmeasurements of a liquid crystal module 104 because the angulardependency is removed. The distance between photodetector 210 and liquidcrystal module 104 is no longer critical because a uniform cone angle isalways sampled. In a preferred embodiment, elastomeric or foam cushion212 is included in sensor system 102 to block out ambient light whichmight affect the accuracy of the measurement. Cushion 212 also uniformlydistributes the contact pressure of housing 202 out and away from themeasuring spot at which sensor system 102 is aimed to reduce cell gapchanges and sheer forces which might disturb the anchoring of liquidcrystal modules. After a calibration operation, sensor system 102 can beused in the off contact mode to measure the amount of veiling glarecaused by the ambient light. This is particularly useful in imageproofing operations for digitally created content where the contentcreator and the publisher or printer reside in greatly differing ambientlight environments. Sensor system 102 is also operable in off contactmode for use to calibrate individual sections of large video walls wherecontact measurements may be physically difficult to obtain.

During the addressing process for Thin Film Transistor (TFT) LCDs, asignal voltage is applied to the source electrode to be transmittedthrough the drain electrode to the liquid crystal layer and a capacitor.Even though the function of this capacitor is to dissipate its chargethroughout the remainder of the frame cycle, there is still some 10% to15% decay of the magnitude of the charge. This results in a slightnon-uniformity of the transmission level (or grayscale level or “color”)of the pixels during a cycle. It is this instability or “frame flicker”that can be interpreted as “noise” in other types of calorimetricdevices. As a reference, this effect is much more pronounced in CRTswhere there is a 100% decay in the light emission of a pixel for thegreater part of a frame cycle.

FIG. 3 shows the schematic diagram of the electronics of circuit board214. Such electronics include a microcomputer 300, a RS 232 or USB port304, an Analog/Digital (A/D) converter 320, voltage regulators 306 and308, filter circuit 302, and photodetector 210. Microcomputer 300preferably has an 8 bit wide data path and 14 bit wide instructions thathandle signals from and commands to photodetector 210. A/D converter 320preferably has a 4 channel 8 bit implementation and sends the 0V to 5Vanalog signals from filter circuit 302 to microcontroller 140 to hostcomputer 106 over the RS 232 or USB port. Microcomputer 300 can act as apass through between host computer, panel computer, and itself.

Errors are inherent to all digital sampling systems from signals withfrequencies above the sampling rate of the Analog/Digital (A/D)converter 320. This “aliasing” phenomena can make the signal appear as alow frequency distortion if left unfiltered and cannot be removed bypost-acquisition processing. Filter circuit 302 is applied to the signalfrom the photodetector 210 prior to the digitizing process to perform ananti-aliasing function and secure the accuracy of the signal. In orderto maximize the stability and accuracy of the sensor system 102, theelectrical circuit design of circuit board 214 incorporates variouselements into filter circuit 302. Two operational amplifiers (“op amps”)310 and 312 and several resistors and capacitors are used to form anactive low pass network “Bessel” filter. The feedback loop includes twogain stages that are routed back into filter circuit 302 to produce asharp cutoff with minimum phase distortion, a fast setting time, and avery high rejection ratio. This allows filter circuit 302 to stabilizethe signals from photodetector 210 and effectively deal with whateverflicker is in liquid crystal module 104. The 4-pole Bessel filterprovided by filter circuit 302 eliminates errors by removing signalswith frequencies above the A/D sampling rate. Filter circuit 302 has amonotonically decreasing magnitude response and is characterized by analmost constant group delay across the entire bandwidth in order topreserve the wave shape of the signals being filtered. Insensitive toenvironmental changes and aging, filter circuit 302 preferably has an 80db cutoff at 60 Hz.

In a preferred embodiment, large dynamic range micropower dual op amps310 and 312 are used that feature a high bandwidth-to-power consumptionratio with true rail-to-rail inputs and outputs. Op amps 310 and 312 canachieve a 200 kHz gain-bandwidth product and are unity-gain stable whiledriving any capacitive load and do not suffer from midswingcommon-mode-rejection degradation or crossover nonlinearity. A seriesresistor 314 and a single-turn potentiometer 316 for fine tuning thesignal range are placed between the negative input and output of op amp310 while another resistor 318 is placed between the negative input ofop amp 310 and ground. These elements collectively control the range ofsignals from the photodetector 210 to A/D converter 320 in the followingmanner: Resistor 314 divided by Resistor 318 plus 1 sets the MinimumGain, while (Potentiometer 316 plus Resistor 314) multiplied by Resistor318 plus 1 sets the Maximum Gain. By this method, photodetector 210 maybe calibrated over a broad range of frequencies with an attenuation of17-bits which is equivalent to a signal-to-noise ratio of 10,000decibels (dB) down.

The flat panel liquid crystal module 104 is typically illuminated by twosets of colored lamps, identified as Source 1 (Red) and Source 2 (Blue),each with a different spectral output. The light from each of thesesources is filtered by Red, Green, and Blue absorption filters withinliquid crystal module 104. Illumination from Source 1 produces a whitepoint (R=G=B=255) that has a correlated color temperature ofapproximately 3,650 K. Illumination from source 2 produces a white pointcolor temperature of approximately 12,300 K. Sensor system 102 allowsboth the screen luminance and the color temperature to be independentlyset by adjusting the intensity of each illumination source. The whitepoint of liquid crystal module 104 can theoretically be set to a rangeof color temperatures from 3,600 K to 12,000 K. Over this restrictedrange of Daylight white illuminants, a vector representation of theinverse of the correlated color temperature has been determined thatvaries linearly with the chromaticity coordinates. The calibration andsetup of a dual primary monitor can be simplified for sources with verystable chromaticities. The output of each source need not be absolute ifits chromaticity is stable. Such stability enables the use of thepresent sensor design that is calibrated for the Green channel where thebandgap energy is most stable.

FIG. 4 shows the relationship of various hardware and softwarecomponents to sensor system 102 in a cross platform solution embodiment.Calibration software module 172 resides in host computer 106 connectedthrough graphics card 134 in flat panel monitor 108 via video interfacecable 183 and is platform independent. Calibration software module 172provides a graphical user interface 170 to liquid crystal module 104residing within flat panel monitor 108 to prompt the user to initiatethe generation a series of gray scale splash screens 171 used in thecalibration operation. Calibration software module 172 passes controlinformation through sensor application programming interface 174 tosensor driver module 176 to instruct sensor system 102 to measure theluminance level of each splash screen generated on liquid crystal module104. Sensor system 102, with the aid of onboard firmware, transmits theluminance levels of the gray scale splash screens from liquid crystalmodule 104 through either a serial interface 178, an I²C interface 180,or a USB interface 182 to calibration software module 172. Uponcompletion of the diagnostic portion of the calibration session,calibration software module 172 provides control information fromgraphics device 134 through backlight application programming interface175 to backlight driver 177 residing within flat panel monitor 108 overthe I²C interface portion 180 of video cable 183 to set the appropriatewhite balance and luminance levels on the backlight of liquid crystalmodule 104. The RGB chromaticity information from this calibrationoperation can then be used to calculate the proper gamma values to loadinto lookup tables for flat panel monitor 108. All the luminance, whitebalance, and gamma information can also be saved as an InternationalColor Consortium (ICC) monitor profile with Adobe Photoshop andcommunicated electronically to other monitors and used to calibrate themas well in a Master/Slave relationship. This embodiment is to makeavailable the differentiated features of specific flat panels types on awide variety of video cards without being dependent on each card vendorto enable the specific features or confronting the user with a difficultOSD challenge. This software would also provide a consistent userexperience, facilitating the addition of more features over time.

In order to function as an absolute luminance sensor, sensor system 102is calibrated using a known transfer standard. An example of a knowntransfer standard is a Minolta CS-100 Colorimeter. A flat panel displaywith adjustable luminance and a Colortron Spotlight Precision LightTable are also used in the process as the light sources. The luminanceresponse of the CS-100 Colorimeter has been set by the manufacturer to astandard lamp and its chromaticity coordinates have been adjusted usinga spectral radiation standard lamp at 3111° K calibrated by the ElectroTechnical Laboratory of the Japanese Ministry of International Trade andIndustry. In a preferred embodiment, a display uses a red and blue lamppair for illumination. The intensity of each lamp pair may be changed toset the color temperature and luminance of the display. The calibratedtarget luminance for the display is 200 Candelas/m² and the target colortemperature is a Daylight white modified Blackbody temperature of D55.It is preferable to achieve the luminance target than the colortemperature target.

After achieving the appropriate targets, red and blue signals are set tozero so that only the green signals are applied to photodetector 210.The potentiometer P1 in filter circuit 302 is adjusted to achieve avalue of 235 counts from A/D converter 320. The voltage at the output ofamplifier 310 is checked to make sure it is in saturation. A 0.3 neutraldensity is placed in front of photodetector 210 to determine whether thereading drops to 117 or 118 counts for verification of saturation. Ifthe signal does not drop, photodetector 210 has too much light fallingon it and the neutral density filtration must be increased. If increasedfiltration is needed, all previously selected trial samples arerecalibrated. The count for the Red and Blue screens are then checked,and if they exceed 240, the spectral filtration is decreased in the Redand Blue spectral regions. If the count falls below 128, the spectralfiltration is increased in the appropriate region. Once sensitivitiesare set to the proper operating ranges, sensor system 102 is ready forgeneral production.

Along with calibration software module 172 in host computer 106,calibration software module 172 is operable to calibrate liquid crystalmodule 104 by setting its luminance, color temperature, and gammaprofile. The task of calibration software module 172 is to determine thecolor temperature of the flat panel liquid crystal module 104 inresponse to luminance measured by microcontroller 140, whose lamps donot linearly track with the drive signal, by using only a spectrallynon-selective photodetector 210. Calibration software module 172measures the gamma or luminance response of liquid crystal module 104and can store information in an International Color Consortium (ICC)profile used by the International Color Microcode standard for colorencoding. Since the lamp drivers are nonlinear, the initial state of thesystem is unknown. Calibration software module 172 uses the color mixinglaw and provides a conversion for the relative color vectors tochromaticity coordinates in order to permit a determination of therelative contributions of the Red, Green, and Blue channels of luminanceSource pair 1 and luminance Source pair 2. The tristimulus valuesproduced by the Red, Green, and Blue color filters are stable.Therefore, the calorimetric output of each individual filter of liquidcrystal module 104 can be determined.

The initial state and the signal linearity (from the alternating current(AC) lamp inverter power source) can be determined in the followingmanner. First, the illumination levels for both Source 1 and Source 2are set to the minimum and the signal level is recorded. Second, theillumination output from Source 1 is increased in four steps from theminimum to the maximum and the signal levels are recorded. Next, theillumination output from Source 2 is increased in four steps from theminimum to the maximum and the signal levels are recorded. Finally, thedifference in the signals may be used to map the outputs of the twosources as a function of the input signal. From the starting tristimulusvalues, the initial and differential state of the illumination sourcescan be determined. Appendix A shows an example of the calibrationdetermination. Once calibration software module 172 in host computer 106along with microcontroller 140 has made this determination, calibrationsoftware 172 can set the luminance and color temperature of the liquidcrystal module 104. Given a required correlated color temperature, thechromaticity coordinates for that point as well as the necessary lampdrive voltages for a required luminance can be determined based on thefact that each illumination source contributes a known proportion of thecolorimetry to the mixture. Additionally, adjustments to the displaycalorimetric settings may be made by bypassing the graphics card 116 inhost computer 106 and accessing microcontroller 140 directly by means ofmanually interacting with OSD board 126. In a preferred embodiment, auser is able to provide input to calibration software module 172 andreceive output from microcontroller 140 by means of GUI 170. Calibrationsoftware module 172 should also provide information by means of GUI 170to enable a user to generate an International Color Consortium (ICC)profile for liquid crystal module 104, preset calorimetric values, andcreate and store custom user settings. Calibration software module 172can also calculate correlated color temperature and report brightness toplus or minus two percent. Calibration software module 172 integratesover several cycles for improved stability.

Sensor system 102 has an optical path with spectral filter elements thatprovide the proper calorimetric properties that linearly correlate astandard signal with the luminance of liquid crystal module 104 in theGreen spectral region. From this correlation, a relationship can beestablished with the other primary channels. Using a spectrallynonselective photodetector, the chromaticity of the primary colorfilters in liquid crystal module 104 can be determined from measuringtheir luminance. Moreover, by calibrating the white point, all the grayscale points and RGB color primaries are matched as well. Since thebacklight is common to all colors in liquid crystal module 104 andvaries in a known stable way within the whole range, setting one primarycolor can effectively set all the others. Thus, setting the gammaresponse curve for white can also set the gamma for the RGB colorchannels. Similarly, setting the gamma response curve for the RGB colorchannels can also set the gamma for all the gray scale levels of white.For a cathode ray tube device, the different primaries may requiredifferent gamma corrections as the setting of one primary color may beaffected by an interaction from the other primary colors.

FIGS. 5A-H show the screen displays provided by graphical user interface170. FIG. 5A shows a preset calibrations window. FIG. 5B shows a detectproperties menu. FIG. 5C shows a customized settings window wherespecific color temperature, luminance, and gamma values may be set bythe user. FIG. 5D shows a window to save custom settings. FIG. 5E showsa calibrate display window where calibration of liquid crystal module104 is initiated. FIG. 5F shows the splash screen windows of red, green,and blue used during calibration. FIG. 5G shows a calibration completewindow. FIG. 5H shows a preferences window.

Sensor system 102 may also be used to correct an on-screen image forunwanted light reflected from the surface of liquid crystal module 104.By using the spectrally nonselective photodetector 210 and color filter216 within sensor system 102 in an off contact mode, a quantitativecorrection factor to the gamma setting of liquid crystal module 104 maybe obtained to maintain the appearance of an image on two or morespatially separated monitors despite the relative ambient viewingconditions.

Liquid crystal module 104 operating in ambient conditions suffers fromthe external lighting in the environment to some degree. This externalluminance can cause stray reflections when it strikes the surface ofliquid crystal module 104 which in turn causes desaturation of thecolorimetry of an image (or even text) as well as adversely affectingits contrast. These stray reflections can lower the gamma profile ofliquid crystal module 104. Sensor system 102, when held approximatelytwenty inches above the surface of liquid crystal module 104, canmeasure the ambient light from liquid crystal module 104 and correct forit by appropriately modifying the gamma response curve of liquid crystalmodule 104 so that the saturation and hue of the original image ismaintained. Calibration software module 172 will accomplish thiscorrection by applying a secondary gamma curve as a transfer functioncorrection. For example, the gamma curve will be increased in the caseof bright ambient conditions. In another preferred embodiment, the gammacorrection can be accomplished by directly adjusting the bias voltagesdriving liquid crystal module 104. Because desaturation from veilingglare is caused by the magnitude of the reflected luminance from liquidcrystal module 104, the white point setting of the dual spectrum lampswill not be affected. Since sensor system 102 integrates all thereflected room light reaching the viewer of liquid crystal module 104, afirst order correction at the veiling glare problem is obtained.Additionally, in the process of designing a filter to equalize theresponse of the photodetector, a major portion of the infrared energy iseliminated, which would affect the accuracy of the veiling glareadjustment. All of this is accomplished with a single non-spectrallyselective photodetector 210.

Video or static images or scenes are created, edited, stored, and thenpresented on media according to the values for hue, saturation, andcolor temperature with which the director of photography, author, oreditor imparts to them. Once they are so imprinted and/or duplicated, nofurther changes to the above identified properties for individualportions of the content are possible without changing those propertiesfor all the content. The viewing experience of any video or stillimagery may be enhanced through the use of dynamic control of the colortemperature of electronic or film media through the use of a dual- ormulti-spectrum lighting source. The content author or editor may store adiscrete track of color temperature values in synchronization withvisual digital media such as Digital Video (DV), High-DefinitionTelevision (HDTV), eCinema, Digital Video Disk (DVD), as well as videostored in QuickTime, AVI, RealVideo, or other formats. It is alsoforeseeable that this method of enhancement would work favorably inother formats including vector animation such as Flash, presentationsoftware such as Microsoft PowerPoint, slide-show software, taggedstatic image file formats such as JPEG or GIF images in web pages,PhotoCD, TIFF, PhotoShop, and other formats.

Different color temperature values may be encoded in a track or othersuitable memory location within a given storage medium that aresynchronized to appropriate scenes or images within the visual contentbeing presented. Calibration software module 172 is operable to controlliquid crystal module 104 and transmit the encoded color temperaturevalues over a video signal cable (DDC), serial interface, UniversalSerial Bus (USB) interface, or other suitable interface protocol. Thisinformation can then be detected by image drivers residing in liquidcrystal module 104 that are capable of having color temperaturecontrolled independently of tristimulus image data via the video signalcable (DDC), serial, USB, or other control mechanism. Some standardshave been developed that encode critical color calibration parametersabout the creation or printing of a given image within the body of datathat describes that image. Flat panel color calibration system 100,however, includes a backlit liquid crystal module 104 with adjustablecolor temperature data wherein color temperature of the liquid crystalmodule 104 can be adjusted in a dynamic manner. Calibration softwaremodule 172 can provide dynamic adjustment of color temperature ofdifferent frames of the displayed image for both playback and authoringenvironments. In addition standard file formats such as TIFF, QuickTime,and others can be extended to include color temperature datainformation.

It should be noted that in addition to the dynamic adjustment of colortemperature, the brightness of liquid crystal module 104 can also besynchronized to the static or video image content being displayed.Calibration software module 172 can be instructed to examine the grayscale content of a frame or series of frames to compute, in oneembodiment, an arithmetic mean of the dynamic brightness level. Actingupon instructions from a given set of parameters, calibration softwaremodule 172 can transmit control information from graphics card 134through backlight application programming interface 175 to backlightdriver 177 to dynamically set the appropriate brightness level forliquid crystal module 104 by controlling the voltage levels to lampsource 1 and lamp source 2 in tandem. In another embodiment, thisinformation may be pre-recorded on a separate information track foraccess during playback of the video content.

It should also be noted that, in concert with the dynamic adjustment ofwhite balance and brightness, calibration software module 172 may alsobe instructed to dynamically adjust the gamma profile of the originalvideo image according to a given set of parameters such as thosecontained in a look-up table memory location within host computer 106 orgraphics card 134. By coincidentally lowering the backlight brightnesslevel of liquid crystal module 104 and decreasing the gamma function sothat the same luminance level is maintained, greater color visual impactmay be realized from a static or video image. In another embodiment, thechange in gamma can be accomplished without loss of gray scaleresolution by dynamically applying and adjusting a set of gammacontrolling voltages to the DC reference circuit of liquid crystalmodule 104 to change its gamma response profile. Because the input videosignals are not affected, the same color resolution and dynamic rangeare maintained.

Flat panel calibration system 100 provides a low cost high performancecapability to calibrate display devices and extend the serviceable lifeof display devices. By being able to adjust a display device to a knownstandard, a content creator, director of photography, andprinter/publisher can effectively consummate a proofing process inminutes over great distances. Color temperature and gamma savvy versionsof DVD players and other software applications may also be developed andaccommodated. While the present sensor system takes advantage ofadjustable white balance and adaptive gamma technology, it can beutilized for both standard LCD and CRT devices as well.

Thus, it is apparent that there has been provided, in accordance withthe present invention, a compact flat panel color calibration systemthat satisfies the above-described advantages. Although the presentinvention has been described in detail, it should be understood thatvarious changes, substitutions, and alterations may be readilyascertainable by those skilled in the art and may be made herein withoutdeparting from the spirit and scope of the present invention as definedby the following claims.

Appendix A

Sensor Calibration Algorithm

[1] Turn Red Lamp ON with the Blue Lamp OFF and record R₁, B₁, G₁

[2] Turn Blue Lamp ON with the Red Lamp OFF and record R₂, B₂, G₂

[3] Define the Color Coordinates for each LampD ₁=128[R ₁ −B ₁)/(R ₁ +G ₁ −B ₁)]D ₂=128[R ₂ −B ₂)/(R ₂ +G ₂ +B ₂)]

[4] The Color Temperature of the Red Lamp is T₁

The Color Temperature of the Red Lamp is T₂

[5] The target value for D (an arbitrary color coordinate) for anarbitrary Color Temperature T is gas as:D=Γ/T+a

where Γ is the slope and a is a constant

[6] Γ=T₁T₂[(D₁−D₂)/T₂−T₁]

[7] a=(D₂T₂−D₁T₁)/(T₂−T₁)

Setting the Color Temperature and Screen Luminance

ASSUMPTION: The Luminance value for Green is a linear function of screenluminance.

[1] From a desired value for T, the target value of D and D_(T) can becalculated fromD=Γ/T

[2] Choose a target value for Screen Luminance, G_(T)

[3] Set both lamps in the center of their respective ranges or at thelast Luminance state if a calibration has just been performed.

-   -   Measure the initial state, R_(I), G_(I), B_(i)Red drive        S_(RI)+Blue drive S_(BI)

[4] Decrement the Red Lamps and measure R_(J), G_(J), B_(J) forS_(RI)−10

[5] Return Red setting to original value and decrement the Blue Lampmeasuring R_(K), G_(K), B_(K) for S_(BI)−10

[6] Calculate G_(I), G_(J), G_(K), D₁, D_(J), D_(K) and form thefollowing derivatives:ΔD _(R)=(D _(I) −D _(J))/10derivateDw/rRedΔD _(B)=(D _(I) −D _(K))/10derivateDw/rBlueΔG _(R)=(G _(I) −G _(J))/10derivateGw/rRedΔG _(B)=(G _(I) −G _(K))/10derivateGw/rBlue

[7] Assuming the system is linear:D _(T) =D _(I) +ΔD _(R)·Δ_(R) +ΔD _(B) ·ΔBG _(T) =G _(I) +ΔG _(R)·Δ_(R) +ΔG _(B) ·ΔB

and

${\begin{matrix}{D_{T} - D_{I}} \\\; \\{G_{T} - G_{I}}\end{matrix}} = {{\begin{matrix}{\Delta\; D_{R}} & {\Delta\; D_{B}} \\\; & \; \\{\Delta\; G_{R}} & {\Delta\; G_{B}}\end{matrix}} \cdot \begin{matrix}\Delta_{R} \\\; \\\Delta_{B}\end{matrix}}$

Solve for Δ_(R)and Δ_(B)which is the amount of the Red and Blue lampsmust be changed, and the new starting value for the initial statebecomesS _(RI New) =S _(RI OLD)+Δ_(R)S _(BI New) =S _(BI OLD)+Δ_(R)

Iterate until the state is reached whereD _(T) −D ₁=0and G _(T) −G _(l)=0

1. A system for calibrating a display, comprising: a lens operable toreceive first and second incoming lights emitted by a display device,the first incoming light not including ambient light, the secondincoming light including ambient light glaring on the display device; aphotodetector operable to detect properties of the first and secondincoming lights regardless of the location of the lens in relation tothe display device; a microprocessor operable to generate a luminancevalue of the display device in response to the properties of the firstand second incoming lights detected by the photodetector, themicroprocessor operable to provide the luminance value in order toadjust display parameters for the display device, the microprocessoroperable to generate the luminance value to compensate for ambient lightglaring on the display device.
 2. The system of claim 1, wherein thelens is operable to pass light in the manner of a total internalreflection 45-degree mirror.
 3. The system of claim 1, furthercomprising: a color filter operable to filter the first and secondincoming lights before it passes to the photodetector.
 4. The system ofclaim 3, wherein the color filter is operable to equalize the first andsecond incoming lights for calibration in a Green spectral region. 5.The system of claim 1, wherein the photodetector is spectrallynon-selective.
 6. The system of claim 5, wherein the photodetector is alinear silicon photodetector.
 7. The system of claim 1, furthercomprising: a software application resident on a host computerassociated with the display device, the software application operable toaccept input from a graphical user interface, the graphical userinterface operable to be displayed on the display device.
 8. Theapparatus of claim 1, further comprising: an elastomeric cushionoperable to position the lens above the display at a distance withoutaffecting accuracy.
 9. The apparatus of claim 1, further comprising: afilter circuit operable to perform an anti-aliasing function on theproperties of the first and second incoming lights detected by thephotodetector.
 10. The apparatus of claim 9, wherein the filter circuitis operable to stabilize the properties of the first and second incominglights detected by the photodetector and preserve the corresponding waveshape.
 11. A compact flat panel color calibration system, comprising:means for detecting a first incoming light emitted from a display, thefirst incoming light not including ambient light; means for detecting asecond incoming light emitted from a display, the second incoming lightincluding ambient light glaring on the display; means for determiningthe color temperature of the first and second incoming lights; means fordetermining the luminance of the first and second incoming lights; andmeans for adjusting emissions from the display in accordance with thedeterminations of color temperature and luminance to compensate forambient light glaring on the display.
 12. The system of claim 11,further comprising: means for emulating a green channel of the display.13. The system of claim 11, further comprising: means for providingchromatic sensitivity; means for setting a proper luminance signalrange.
 14. The system of claim 11, further comprising: means for tuninga signal range of the first and second incoming lights.
 15. The systemof claim 11, wherein the means for detecting the first and secondincoming lights is placed without regard to any distance requirementsfrom the display.
 16. A method of calibrating a display, comprising:receiving a first incoming light emitted by a display, the firstincoming light not including ambient light; receiving a second incominglight emitted by the display, the second incoming light includingambient light glaring on the display; generating signals correspondingto the first and second incoming lights; determining a luminanceassociated with the signals; adjusting emissions from the display inresponse to the luminance to compensate for ambient light glaring on thedisplay.
 17. The method of claim 16, further comprising: filtering thefirst and second incoming lights to emulate a green channel of thedisplay.
 18. The method of claim 16, further comprising: filtering thesignals to remove frequencies above a threshold level.
 19. The method ofclaim 16, further comprising: tuning the signal range of the signals.20. The method of claim 16, further comprising: determining a colortemperature associated with the signals; adjusting emissions from thedisplay in response to the color temperature.